Using a newly developd method for estimating relative levels of IgG1-class and IgG2-class antibodies of a given specificity in a serum, and for converting antisera or globulins to preparations containing only the IgG2 class or IgG1 class among the 7S antibodies of that specificity, it is planned: 1. To determine the relative levels of IgG1-class and IgG2-class antibodies, original and "plateau" levels of suppressive and cytotoxic titers, in antibodies vs. alloantigens in mice of these age groups which had been immunized by allografting, spleen cell injection, or injecting of soluble antigens, to see whether there is a relatively greater deficit of production of alloantibodies of IgG2 class which might account for the prolonged retention of allografts in aging animals. 2. In inbred mice bearing autochthonous tumors, to study the relation between the levels of anti-tumor antibodies of IgG1-class and IgG2-class produced, to see whether there is a special deficit of IgG2-class anti-tumor antibodies which might account for the greater susceptibility of aging mice to transplanted tumors.